In the process of manufacturing semiconductor integrated circuits (ICs), a semiconductor wafer is subjected to various processes, such as film formation, oxidation, diffusion, annealing, modification, and etching. A processing system of the so-called cluster tool type, in which process chambers for performing respective processes are connected to a common transfer chamber, is known as a system for efficiently performing the processes described above. In the processing system of the cluster tool type, wafers are transferred between the process chambers by a transfer apparatus disposed in the common transfer chamber.
FIG. 7 is a schematic plan view showing a conventional processing system of the cluster tool type. As shown in FIG. 7, for example, the processing system 2 has a plurality of, e.g., three in this case, process chambers 8A to 8C, which are connected to a common transfer chamber 4 that can be vacuum-exhausted, respectively via gate valves 6A to 6C. The process chambers 8A to 8C are respectively provided with worktables 10A to 10C disposed therein, for placing a semiconductor wafer W on the top surface. Two cassette chambers 14A and 14B are connected to the common transfer chamber 4 respectively via gate valves 12A and 12B, and configured to accommodate a cassette for storing semiconductor wafers, which are almost circular.
The common transfer chamber 4 is provided with a transfer apparatus 16 disposed therein and formed of, e.g., an articulated arm, which is rotatable and extensible/contractible. The transfer apparatus 16 has a support portion 18 to hold a wafer W, and transfer and deliver it between each of the cassette chambers 14A and 14B and each of the process chambers 8A to 8C, and between the process chambers 8A to 8C. The common transfer chamber 4 is provided with an orienter 17 provided therein, for allowing the transfer apparatus 16 to hold a wafer W in the proper direction and at the proper position.
When the transfer apparatus transfers a wafer W, the wafer W is not necessarily accurately placed at the normal position on the support portion 18 of the transfer apparatus 16. If a wafer W with a misalignment is placed as it is on the worktable of a next process chamber, the process suffers ill effects. Accordingly, it is necessary to correct the misalignment of the wafer W, so that the wafer W is placed on the worktable of the process chamber at the proper position.
A misalignment of a wafer W occurs in the following cases. When the gas pressure in the process chambers 8A to 8C changes, the wafer W may slip on the worktables 10A to 10C. When the wafer W is delivered, the wafer may slip on the worktables 10A to 10C. Where the worktables 10A to 10C are respectively provided with electrostatic chucks, the wafer may be popped by a residual charge, when a wafer W is delivered from the worktables 10A to 10C. This latter phenomenon occurs, if the change on the wafer W has not been sufficiently removed when the wafer W is delivered from the worktables 10A to 10C.
U.S. Pat. No. 5,917,601 (Jpn. Pat. Appln. KOKAI Publication No. 10-223732), Jpn. Pat. Appln. KOKAI Publication No. 10-247681, and U.S. Pat. No. 5,483,138 disclose a misalignment detector to solve the problems described above.
The processing system shown in FIG. 7 also has a conventional misalignment detector. Specifically, in the common transfer chamber 4, a pair of line sensors 20 and 22 are disposed with a certain distance therebetween near each of the gate valves 6A to 6C of the process chambers 8A to 8C. When a wafer W passes between the line sensors 20 and 22, the wafer W is temporarily stopped, and two positions on the edge (positions on the peripheral contour) thereof are detected. On the basis of these detected values, it is possible to obtain how much misalignment exists between the center of the wafer and the normal position on the support portion 18. The control section of the transfer apparatus 16 controls the rotation amount and extension/contraction amount of the transfer apparatus 16 to offset the obtained misalignment amount, so as to place the wafer W on the worktable at the proper position.
This misalignment detector requires a wafer W in transfer to be temporarily stopped at a position corresponding to the line sensors 20 and 22, so as to detect a misalignment amount. For example, the necessary stop time is about 2 to 3 seconds, depending on the performance of the line sensors 20 and 22. Accordingly, the wafer transfer cannot be speeded up, thereby lowering the throughput. This brings about a serious problem, particularly where a wafer is subjected to a number of processes in different process chambers in one processing system 2. This is so, because the wafer in transfer has to be temporarily stopped every time when the wafer is transferred into each of the process chambers, so as to detect a misalignment amount.